$f(x) = \begin{cases} 0 & \text{if } x = 3 \\ -x^{2}+3 & \text{otherwise} \end{cases}$ What is the range of $f(x)$ ?
First consider the behavior for $x \ne 3$ Consider the range of $-x^{2}$ The range of $x^2$ is $\{\, y \mid y \ge 0 \,\}$ Multiplying by $-1$ flips the range to $\{\, y \mid y \le 0 \,\}$ To get $-x^{2}+3$ , we add $3$ If $x = 3$, then $f(x) = 0$. Since $0 ≤ 3$, the range is still $\{\, y \mid y ≤ 3 \,\}$.